Issue #174: Improved pool to reconnect to zk
Issue #174: zk: disabled infinite reconnect
Issue 174: api: handle common exceptions as 500 errors
Issue #174: Fixed closing None-backend
Style fix
Fixed test
Fixed pylint test
Issue #174: Simplified Pool(): removed __enter__()/__exit__(), list with all connections
Issue #174: separate exceptions handling
Pool: removed task_done()
api: fixed 500 for built-in exceptions
Implemented Backend.is_connected()
Default backends connections for api == 1
Issue #174: Pool: separated creating and opening of the backend
Issue #174: Pool: reusing backend objects
Fixed using of format()
tox: comment about logilab-common
Merge pull request #175 from mdevaev/master

Fixed #174: improved pool and api to using zk